Judith Brodie, director of VSO, the international development agency, said many schemes offering placements in developing countries were a form of "charity tourism" and more about enjoyment than helping the host communities.
Gap years abroad have become a rite of passage for young people, and are usually taken in the year before students start university or after graduation. They are seen as a chance to broaden the mind and learn about other cultures while contributing to the community.
But Ms Brodie said: "Some gap year providers seem to pay little attention to whether young people make any long-term difference to the communities they are working in. There seems to be a colonial attitude that assumes that just because a young person is from the UK they will benefit their host community."
She said that in extreme cases volunteers could do more harm than good and become a drain on local resources.
Her comments ring true for some former volunteers including Catherine Harrington, 24. She went to Costa Rica with the Inter-cultural Youth Exchange (ICYE) for six months in 2000. The ICYE run exchanges where British students pay for the experience to help fund reciprocal trips for students from their host country.
When she arrived in Costa Rica, Miss Harrington found no volunteer work had been set up for her, and at the start she had no host family.
She said the family she eventually lived with received none of the cash she paid to the organisation.
When she was found work after a fortnight Miss Harrington was asked to translate UN documents - despite speaking almost no Spanish. She said she was "not at all useful to Costa Rica".
"Gap years are just hedonistic and those who think they can do some good really can't. You're so young. I never thought I could actually offer that much."
However, Hannah Kowzsun, 23, from Eastbourne, said her gap year trip to South Africa was valuable to her and the community she worked in. She spent a year in a township near Limpopo, teaching English, history and Bible studies in a Catholic school.
She said there was a great deal of difference between working as a teacher for a year and "just swanning in for a few months and then abandoning the community".
Annually, an estimated 50,000 teenagers take a year out before university, and a survey published this week by the Year Out Group said Australia had been replaced as the number one gap year destination by South Africa.
Richard Oliver, the group's chief executive, rejected Ms Brodie's claims.
"This is a competitive and commercial business but the countries our members send volunteers to are very happy to have them," he said.
Jennie Smith, UK manager of the ICYE, said: "I agree with some of the concerns that have been raised and believe that it is essential that volunteer placements are organised sensitively and in partnership with the local community.
"But this investment in young people and the promotion of greater inter-cultural understanding is vitally important, particularly in a world where conflict and suspicion between cultures is increasing rather than receding."